1. Field of the Invention
The present invention relates to a motor driving circuit, and, in particular, to a motor driving circuit for driving a brushless motor.
2. Description of the Prior Art
FIG. 1 shows a block diagram of one example of a motor driving circuit in the prior art.
The motor driving circuit 1 in the prior art drives a three-phase brushless motor 2. The motor driving circuit 1 includes a driving signal generating circuit 3 which detects a rotational position of the motor 2 as rotational position signals in accordance with the rotational position of the motor 2, and generates driving signals in accordance with the detected rotational position. The motor driving circuit 1 further includes a pre-driving circuit 4. The driving signals generated by the diving signal generating circuit 3 are provided to the pre-driving circuit 4, and the pre-driving circuit 4 generates pre-driving currents in accordance with the driving signals. The motor diving circuit 1 further includes output transistors Q1, Q2 and Q3 which are turned on and turned off by the pre-driving currents supplied by the pre-driving circuit 4, and, thus, supply currents to the motor 2. The motor driving circuit 1 further includes resistors R1, R2 and R3 which are connected with the bases of the output transistors Q1, Q2 and Q3, and absorb the leakage currents which otherwise flow to the bases of the output transistors Q1, Q2 and Q3.
The three-phase brushless motor 2 includes a stator coil portion 11 which generates a rotating magnetic field extending in the rotation axis directions (arrow A directions), and a rotor 12 which faces the stator coil portion 11, is rotatably set, includes a magnet magnetized in the rotation axis directions (arrow A directions) to have multiple poles, and is rotated as a result of being affected by the rotational magnetic field generated by the stator coil portion 11. (The basic technology of such a three-phase brushless motor is the same as that of motors disclosed in a book `Precision Small Motor Application Method`, supervised by Hiroshi Yamada, edited by the Industrial Investigation Society (Kogyo Chosa Kai), and published by Kaoru Yoshimoto, the first edition of which was published on Jul. 30, 1986. In particular, for example, see pages 51-53, `Axial Flux Type Brushless Motor`, `Sheet Coil` (1) Structure, pages 201-203, `Axial Flux Type Brushless Motor`, `Sheet Coil Motor` and pages 225-257, `Features and Specification of Used Scanner Motor`, `Speed Control of Scanner Motor`.)
The stator coil portion 11 includes a U-phase coil 21, a V-phase coil 22 and a W-phase coil 23, one end of each of the U-phase coil 21, V-phase coil 22 and W-phase coil 23 being connected with the power source voltage Vcc.
FIG. 2 shows a circuit diagram of the pre-driving circuit 4 of the example of the motor driving circuit of the prior art.
The pre-driving circuit 4 includes a current source 31 and transistors Q11, Q12 and Q13. One end of the current source 31 is connected with a power source voltage Vcc, and the other end of the current source 31 is connected with the emitters of the transistors Q11, Q12 and Q13. Using the power source voltage Vcc, the current source 31 generates and supplies the driving currents, in accordance with the load of the motor 2, to the emitters of the transistors Q11, Q12 and Q13.
The transistors Q11, Q12 and Q13 are PNP transistors, the emitters thereof being connected with the power source voltage Vcc, the collectors thereof being connected with the bases of transistors Q1, Q2 and Q3 through a current amplifier 32, the bases thereof having the driving signals PU, PV and PW supplied thereto by the driving signal generating circuit 3.
The transistor Q11 is a transistor which generates the pre-driving current IPU for U-phase driving, the base thereof having the U-phase driving signal PU supplied thereto from the driving signal generating circuit 3, is turned off and turned on in accordance with the U-phase driving signal PU, and thus controls the pre-driving current IPU to be supplied to the base of the transistor Q1. When the voltage of the U-phase driving signal PU is higher than a predetermined voltage, the transistor Q11 is turned off, thus stopping supply of the pre-driving current IPU to the base of the transistor Q1. When the voltage of the U-phase driving signal PU is lower than the predetermined voltage, the transistor Q11 is turned on, thus supplying the pre-driving current IPU to the base of the transistor Q1.
The transistor Q12 is a transistor which generates the pre-driving current IPV for V-phase driving, the base thereof having the V-phase driving signal PV supplied thereto from the driving signal generating circuit 3, is turned off and turned on in accordance with the V-phase driving signal PV, and thus controls the pre-driving current IPV to be supplied to the base of the transistor Q2. When the voltage of the V-phase driving signal PV is higher than a predetermined voltage, the transistor Q12 is turned off, thus stopping supply of the pre-driving current IPV to the base of the transistor Q2. When the voltage of the V-phase driving signal PV is lower than the predetermined voltage, the transistor Q12 is turned on, thus supplying the pre-driving current IPV to the base of the transistor Q2.
The transistor Q13 is a transistor which generates the pre-driving current IPW for W-phase driving, the base thereof having the W-phase driving signal PW supplied thereto from the driving signal generating circuit 3, is turned off and turned on in accordance with the W-phase driving signal PW, and thus controls the pre-driving current IPW to be supplied to the base of the transistor Q3. When the voltage of the W-phase driving signal PW is higher than a predetermined voltage, the transistor Q13 is turned off, thus stopping supply of the pre-driving current IPW to the base of the transistor Q3. When the voltage of the W-phase driving signal PW is lower than the predetermined voltage, the transistor Q13 is turned on, thus supplying the pre-driving current IPW to the base of the transistor Q3.
The transistors Q1, Q2 and Q3 are NPN transistors, the collectors thereof being connected with the other ends of the U-phase, V-phase and W-phase coils 21, 22 and 23, and the emitters thereof being grounded. The pre-driving circuit 4 supplies the pre-driving currents IPU, IPV and IPW to the bases of the transistors Q1, Q2 and Q3. The transistors Q1, Q2 and Q3 control, in accordance with the pre-driving currents IPU, IPV and IPW supplied from the pre-driving circuit 4, the driving currents IU, IV and IW to be supplied to the U-phase, V-phase and W-phase coils 21, 22 and 23 from the power source voltage Vcc.
The transistor Q1 controls the driving current IU supplied to the U-phase coil 21. When the pre-driving current IPU is supplied to the base of the transistor Q1, the transistor Q1 is turned on, forms a path from the power source voltage Vcc to the ground GND, and supplies the driving current IU to the U-phase coil 21. When the supply of the pre-driving current IPU from the pre-driving circuit 4 to the base of the transistor Q1 is stopped, the transistor Q1 is turned off, cuts the path from the power source voltage Vcc to the ground GND, and stops the supply of the driving current IU to the U-phase coil 21.
The transistor Q2 controls the driving current IV supplied to the V-phase coil 22. When the pre-driving current IPV is supplied to the base of the transistor Q2, the transistor Q2 is turned on, forms a path from the power source voltage Vcc to the ground GND, and supplies the driving current IV to the V-phase coil 22. When the supply of the pre-driving current IPV from the pre-driving circuit 4 to the base of the transistor Q2 is stopped, the transistor Q2 is turned off, cuts the path from the power source voltage Vcc to the ground GND, and stops the supply of the driving current IV to the V-phase coil 22.
The transistor Q3 controls the driving current IW supplied to the W-phase coil 23. When the pre-driving current IPW is supplied to the base of the transistor Q3, the transistor Q3 is turned on, forms a path from the power source voltage Vcc to the ground GND, and supplies the driving current IW to the W-phase coil 23. When the supply of the pre-driving current IPW from the pre-driving circuit 4 to the base of the transistor Q3 is stopped, the transistor Q3 is turned off, cuts the path from the power source voltage Vcc to the ground GND, and stops the supply of the driving current IW to the W-phase coil 23.
FIGS. 3A, 3B, 3C and 3D show operation waveforms of the example of the motor driving circuit in the prior art. FIG. 3A shows the waveforms of the driving signals PU, PV and PW generated by the driving signal generating circuit 3. FIG. 3B shows the waveform of the driving current IU supplied to the U-phase coil 21. FIG. 3C shows the waveform of the driving current IV supplied to the V-phase coil 22. FIG. 3D shows the waveform of the driving current IW supplied to the W-phase coil 23.
First, the voltages of the driving signals PV and PW supplied from the driving signal generating circuit 3 to the pre-driving circuit 4 increase and the voltage of the driving signal PU decreases. From the time t1, as shown in FIG. 3A, the voltage of the driving signal PU is lower than the voltages of the driving signals PV and PW. Thereby, in the pre-driving circuit 4, the transistor Q11, to which the driving signal PU is supplied, is turned on, and the transistors Q12 and Q13, to which the driving signals PV and PW are supplied, are turned off.
When the transistor Q11 is turned on, the pre-driving current IPU is supplied to the base of the transistor Q1, and thereby, the transistor Q1 is turned on. On the other hand, because the transistors Q12 and Q13 are turned off, the pre-driving currents IPV and IPW are not supplied to the transistors Q2 and Q3. As a result, the transistors Q2 and Q3 are turned off.
When the transistor Q1 is turned on and the transistors Q2 and Q3 are turned off, the other end of the U-phase coil 21 is connected to the ground GND, and, as shown in FIG. 3B, the driving current IU flows through the U-phase coil 21. When the driving current IU flows through the U-phase coil 21, the magnetic field is generated, and thereby, the rotor 12 is rotated by the interaction between the magnetic field generated by the U-phase coil 21 and the magnetic fields generated by the corresponding portion of the magnet of the rotor 12.
Then, the voltage of the driving signal PU increases and the voltage of the driving signal PV decreases. When the voltage of the driving signal PU is higher than the voltage of the driving signal PV from the time t2, as shown in FIG. 3A, the transistor Q11, to the base of which the driving signal PU is supplied, is turned off, and the transistor Q12, to the base of which the driving signal PV is supplied, is turned on, in the pre-driving circuit 4. When the transistor Q11 is turned off, the supply of the pre-driving current IPU to the transistor Q1 is stopped, and thereby the transistor Q1 is turned off. When the transistor Q1 is turned off, the supply of the driving current IU to the U-phase coil 21 is stopped.
Further, when the transistor Q12 is turned on, the pre-driving current IPV is supplied to the base of the transistor Q2, and thereby, the transistor Q2 is turned on. When the transistor Q2 is turned on, the other end of the V-phase coil 22 is grounded, and thereby, as shown in FIG. 3C, the driving current IV is supplied to the V-phase coil 22. As a result, the rotor 12 is rotated by the interaction between the magnetic field generated by the V-phase coil 22 and the magnetic field generated by the corresponding portion of the magnet of the rotor 12.
Then, the voltage of the driving signal PV increases and the voltage of the driving signal PW decreases. When the voltage of the driving signal PV is higher than the voltage of the driving signal PW from the time t3, as shown in FIG. 3A, the transistor Q12, to the base of which the driving signal PV is supplied, is turned off, and the transistor Q13, to the base of which the driving signal PW is supplied, is turned on, in the pre-driving circuit 4. When the transistor Q12 is turned off, the supply of the pre-driving current IPV to the transistor Q2 is stopped, and thereby the transistor Q2 is turned off. When the transistor Q2 is turned off, the supply of the driving current IV to the V-phase coil 22 is stopped.
Further, when the transistor Q13 is turned on, the pre-driving current IPW is supplied to the base of the transistor Q3, and thereby, the transistor Q3 is turned on. When the transistor Q3 is turned on, the other end of the W-phase coil 23 is grounded, and thereby, as shown in FIG. 3D, the driving current IW is supplied to the W-phase coil 23. As a result, the rotor 12 is rotated by the interaction between the magnetic field generated by the W-phase coil 23 and the magnetic field generated by the corresponding portion of the magnet of the rotor 12.
Thus, a rotating magnetic field is generated in the stator coil portion 11, from the U phase to the V phase, to the W phase, to the U phase, . . . The interaction between the rotating magnetic field and the magnetic field of the magnet of the rotor 12 causes the rotor 12 to rotate.
That is, the transistors Q1, Q2 and Q3 are alternately turned on for each 1/3 period at the hatched portions of the driving signals PU, PV and PW shown in FIG. 3A. When the voltage of the driving signal PU is compared with the voltages of the driving signals PV, PW during the period in which the driving current IU does not flow through the U-phase coil 21, and when the voltage difference between the voltage of the driving signal PU and the voltage of the driving signals PV, PW is a minimum one, for example, when the voltage difference is 80 mV, the ratio of the collector currents of the transistors Q11, Q12 and Q13 is (1/20) :1:1.
The driving signal generating circuit 3 generates the driving signals having a fixed amplitude without regard to the load of the motor 2. However, as the load of the motor 2 increases, the driving currents IU, IV and IW increase. The collector currents of the transistors Q11, Q12 and Q13 are set so that each of the collector currents of two of the transistors Q11, Q12 and Q13 which are in their turned off states is 1/20 the collector current of the other one of the transistors Q11, Q12 and Q13. When the driving currents increase in the same ratio, the collector currents of two of the transistors Q11, Q12 and Q13 which should be in their turned off states also increase. Thereby, the base currents are supplied to the two of the transistors Q1, Q2 and Q3 which should be in their turned off states. Thereby, the two of the transistors Q1, Q2 and Q3 which should be in their turned off states are turned on. Thereby, leakage currents flow through the two of the coils 21, 22 and 23 corresponding to the two of the transistors Q1, Q2 and Q3 which should be in their turned off states. Thus, in the time period in which one of the coils 21, 22 and 23 should drive the rotor 12, currents also flow through the other two of the coils 21, 22 and 23. Thereby, an unnecessary magnetic field is generated, and the driving efficiency of the motor 2 is degraded.
In order to prevent generation of such leakage currents of the transistors Q1, Q2 and Q3, resistors R1, R2 and R3 for absorbing the leakage currents are provided between the bases of the transistors Q1, Q2, Q3 and the ground GND. By providing the resistors R1, R2 and R3 between the bases of the transistors Q1, Q2, Q3 and the ground GND, even when the leakage currents are supplied to the collectors of two of the transistors Q11, Q12 and Q13 which should be in their turned off states, the currents flow through the corresponding two of the resistors R1, R2 and R3 to the ground GND. Thereby, increase of the base voltages of the corresponding two of the transistors Q1, Q2 and Q3 due to the leakage currents can be controlled, and the corresponding two of the transistors Q1, Q2 and Q3 are prevented from being turned on.
However, in the motor driving circuit in the prior art, the resistors R1, R2 and R3 are provided between the bases of the transistors Q1, Q2, Q3 and the ground GND so that the leakage currents flow into the ground GND through the two of the resistors R1, R2 and R3. Thereby, when the load of the motor increases and the leakage currents flowing from the bases of the two of the transistors Q11, Q12 and Q13 to the collectors thereof increase, the voltages appearing across the two of the resistors R1, R2 and R3 increase. When the increase of the leakage currents cause the voltages appearing across the two of the resistors R1, R2 and R3 to reach the voltage by which the transistors Q1, Q2 and Q3 are turned on, the corresponding two of the transistors Q1, Q2 and Q3 are turned on. Thereby, as indicated by broken lines in FIGS. 3B, 3C and 3D, the leakage currents flow through the corresponding two of the coils 21, 22 and 23. Thereby the driving efficiency is degraded.
It may be attempted to absorb the leakage current sufficiently by using resisters R1, R2 and R3 having smaller resistances. However, by using the resistors R1, R2 and R3 having smaller resistances, the voltage by which the transistors Q1, Q2 and Q3 are turned on decreases. Therefore, it is necessary to increase the driving currents, and thereby, ineffective currents increase.